1. Field of the Invention
The present invention relates to a sheet-type piezoelectric actuator that includes a plurality of piezoelectric layers stacked on each other.
2. Discussion of Related Art
There is known an on-demand-type ink jet printer head that employs the above-indicated piezoelectric actuator. The known printer head additionally employs a cavity unit having an ink chamber to which a portion of the piezoelectric actuator is opposed. When that portion of the piezoelectric actuator is deformed, i.e., expanded, the volume of the ink chamber is decreased and a droplet of ink is ejected from a nozzle communicating with the ink chamber. Japanese Patent Publication P2001-162796A or its corresponding U.S. Pat. No. 6,575,565 discloses such an example of the piezoelectric actuator that includes a number of piezoelectric layers each of which consists of a sheet formed of a piezoelectric material and which are stacked on each other; and a number of electrode layers each of which is formed on a surface of a corresponding one of the piezoelectric layers. When an electric voltage is applied to the electrode layers, the stacked piezoelectric layers are deformed, i.e., expanded and contracted.
The piezoelectric actuator disclosed by the above-identified documents includes nine stacked piezoelectric layers in total. The lower, six piezoelectric layers, located on the side of the cavity unit, are stacked alternately with six electrode layers including three anode layers and three cathode layers that are also stacked alternately with each other. Each of the anode layers includes a plurality of individual electrodes corresponding to a plurality of ink chambers of the cavity unit, respectively; and each of the cathode layers is common to all the ink chambers. A plurality of portions of each of the piezoelectric layers that correspond to the ink chambers, respectively, and that are sandwiched by the individual electrodes of a corresponding one of the anode layers and a plurality of portions of a corresponding one of the cathode layers are polarized to provide a plurality of active portions of the piezoelectric actuator. Thus, the piezoelectric actuator has an active layer including the same number of active portions as the number of the ink chambers of the cavity unit, and is stacked on the cavity unit such that the active portions of the piezoelectric actuator are located above the ink chambers of the cavity unit, respectively.
Meanwhile, the upper, three piezoelectric layers of the piezoelectric actuator provide a restrictive layer that is stacked on the active layer and is not deformed, i.e., is not expanded or contracted. A plurality of portions of the restrictive layer that are located above the active portions of the active layer, respectively, provide a plurality of restrictive portions that restrict respective upward deformations of the active portions and thereby enhance respective downward deformations of the active portions, i.e., respective deformations of the active portions toward the ink chambers of the cavity unit.
Thus, in the piezoelectric actuator disclosed by the above-identified documents, when the electric voltage is applied to the anode and cathode electrodes corresponding to a desired one of the ink chambers, the piezoelectric layers sandwiched by those electrodes are efficiently expanded and contracted toward and away from the desired ink chamber, so that the volume of that ink chamber is decreased and a droplet of ink is ejected from the nozzle communicating with the ink chamber.
Meanwhile, a piezoelectric layer is formed of a ceramic material that is a dielectric material. Therefore, when a potential difference is produced between two electrodes that are opposed to each other to sandwich the piezoelectric layer, the piezoelectric layer has an electrostatic capacitance. In particular, the above-indicated piezoelectric actuator needs, for the purpose of producing an appropriate amount of displacement thereof to be able to eject ink, a plurality of piezoelectric layers that are stacked alternately with a plurality of electrodes. Thus, the piezoelectric actuator as a whole has a great electrostatic capacitance. An electric power consumption of the piezoelectric actuator is expressed as follows: (Electric Power Consumption)∝(Electrostatic Capacitance)×(Drive Voltage Applied To Piezoelectric Actuator)2. Thus, the electric power consumption increases in proportion to the electrostatic capacitance. Since the increasing of the electric power consumption leads to increasing of the cost of a power supply device of the ink jet printer, there has been a demand to decrease the electrostatic capacitance of the piezoelectric actuator.
Electrostatic capacitance is expressed as follows: (Electrostatic Capacitance)=(Dielectric Constant)×(Area Of Each Of Electrodes)/(Distance Between Electrodes). Thus, the electrostatic capacitance can be decreased by increasing the distance between the two electrodes or decreasing the area of each of the electrodes. However, if the distance between the two electrodes is just increased or the area of each of the electrodes is just decreased, then the amount of displacement of the piezoelectric actuator is also decreased and accordingly the amount of change of volume of the ink chamber is decreased, so that the speed of ejection of ink from the nozzle is adversely influenced. Thus, there has been a need to decrease the electrostatic capacitance of the piezoelectric actuator while keeping the amount of displacement thereof sufficient to eject the ink.